Search This Blog

Friday, 28 February 2014

Above I have stolen the mixer part from the Minima schematic and labelled the ports. In my test setup I have the LO connected to a test crystal oscillator I had lying about, it's running at 20MHz (ish), the RF port is connected to my bench signal generator and the IF port is connected to the Spectrum Analyser.

Here's what the test setup looks like:

Now, the author of the Minima Farhan gave us some information on the KISS mixer here:

Now, here is what I see when I have a play. I have 20MHz LO, 15MHz set into the RF port from my bench sig gen and the spectrum analyser connected to the IF port.

Here's the output spanning 0 through 100MHz with just the LO (20MHz) signal:

This is just my oscillator and it's harmonics - so far so good. Now lets add in the 15MHz signal to the RF port and keep the 100MHz span:

So now we see the 20MHz signal plus harmonics, the 15MHz signal plus harmonics plus all the differences and sums which land at every 5MHz interval. So far so good.

Now, lets zoom in on one of the mixed signals at 35MHz:

Looks good to me.

Now, if I connect my x10 'scope probes to the gates of the FETs, here is what we see:

The 'scope struggles to trigger on anything here, because there is all sorts of frequencies in the signal, however, I think we can safely say that the two signals are the same but out of phase with each other as we would expect.

Now, if I look at the drains (my sources are connected together and the bias supply), we see this:

and if I add the LPF in the scope to bandwidth restrict the inputs:

If we replace the SA on the IF port with a 50R load, then look at the signals on the drains with the SA, here's drain one:

and here is drain two:

Now, in all of the tests above I have tried altering the bias pot and I can see no difference at all under any circumstances. If I look at the voltage at the wiper of the pot I can see 0-5V as expected. However the voltage at the sources read from -0.12 to -0.15V. That isn't what I expected either! If I remove the +5V supply the output doesn't change nor does it if I connect the sources to ground.

The FETs in the KISS mixer effectively replace the diodes in a more traditional diode ring mixer; this chops the RF signal thus providing the IF signal at the centre tap of the RF transformer.

So I replaced the single bias pot with one on each FET, under these circumstances I can alter the amplitude of the unwanted signals in the output but not make any difference to the wanted signals.

In my case the wanted signal is the 20 + 15MHz, here is the mixer output with the two bias pots adjusted to minimise the unwanted signals:

However, if I just remove the 5V line, the bias pots and just ground the two sources on the FETs, here's the output:

And these are the signals on the drains with no bias (the sources grounded):

Friday, 21 February 2014

I've been fiddling some more with my BITX40. I've nearly completed the exciter board and have been playing with the set up of the BFO and the crystal filter. This is where a spectrum analyser really comes into it's own.

Initially I modified the BFO and removed the inductor - the purpose of the inductor is to lower the BFO below the crystal frequency - this is needed in the BITX20 as we need to retain the USB. As I want to keep the LSB in my 7MHz radio, I need the BFO to be higher than in an USB rig.

Here's the carrier or BFO output (in purple) on top of the save I did of the crystal filter sweep - now I don't much like the dip in the top of the Filter sweep, but I can come back to that later.

Now, here the BFO adjustment is at it's lowest frequency and I looked very much to me like this wasn't low enough. Here is the same thing with 1KHz of audio mixed in too:

So, to move the BFO down a bit I have added in the 15pf capacitor in the original design which is in parallel with the trimmer adjustment of the carrier:

Now, this looks close to perfect to me.

So I adjusted the balanced modulator to get as much carrier suppression as I could:

I think this could be way better if I actually matched some diodes, but I don't think it's going to matter a whole pile. Now if I look at the signal on the output side of the crystal filter with all these adjustments made it looks like this:

Which I reckon is as near to perfect as I am going to get.

Here's a recent picture of the board:

So now I need to move and test further into the circuit. We now need to take this 10MHz output of the filter and mix it with my VFO to make the 7MHz LSB signal I am after. So here is the output of the Band Pass Filter in TX:

For my version which I am aiming at 40M, I need to change the VFO. Mine is going to tune from 2.8MHz to 3MHz. I did some sums and calculated that I needed about 22uH of inductance. Now that's quite a lot!

I ended up winding 62 turns onto a T68-2. Here's the VFO part of the circuit built:

Now, the FLL/counter comes already built. It connects to the output of Q17 - when I connect there I get no results from the counter, but when I connect to Q6 it works fine. The output of Q17 looks like this on the 'scope:

The waveform at Q6 is the same, just slightly larger amplitude - strange - almost like there isn't enough drive at the FLL/Counter connector. I've decided to worry about this later and move on.

Sunday, 16 February 2014

Following a whistle-stop tour of the Middle East, I am back and have done some more with my Minima. After much deliberation and head scratching, I have decided to box the linear amplifier and Low Pass Filters separately. This way I can re-use the amp later for other projects should I decide to do so.

So here's the finished Minima:

You will see that I have used a DIN socket to take the BCD band selection information from the processor to the rear panel, this will allow me to automatically switch the LPFs in the linear as the radio tunes. I have also added a simple PTT line to the rear panel to switch the linear into TX.

Here's the linear and filters in a state of incompleteness:

So, the BCD output is configured in accordance with this table:

I've included 6 low pass filters with the frequencies listed in the column TX LPF above. So as the radio tunes when the frequency passes one of the thresholds the output binary value is set on the three lines I have attached to the DIN socket. The only thing to note is that the 19MHZ TX filters is engaged with BOTH the 15MHz and 30MHz TX filter. This has meant I have to use a simple diode matrix on the output of the 4028 CMOS chip to enable the same filter on two different outputs. So in the linear box we have the 4028 CMOS BCD converter plus a DIL package with darlington pairs to physically switch the relays.

If you need a schematic of the linear control wiring that takes the BCD signals from the processor and switches the LPFs email me and I will create one.

I've made a few modifications to the software, including the logic needed for my LPF switching in the linear. Rather than explain all the details I have simply uploaded my code here:

Since its publication I have received correspondence from far and wide on the project (all positive I have to say!), but this is the best so far. Alex, AI2Q has made the project and enclosed it in an old Heathkit enclosure (he is an ex Heathkit engineer). I thought this was fantastic: